The present invention relates to a light fiber and a method of producing the same and, more particularly, to a light fiber comprising (a) a tubular clad having a predetermined length, which is expandable under pressure and (b) a solid core formed by reacting a filler material with which the clad is filled, the clad and the core being closely contacted with each other by shrinkage of the clad, and a method of producing the same, particularly a method of producing a light fiber, capable of effectively preventing generation of air gap between the clad and the core.
There has been known a method of filling a tubular clad with a polymerizable monomer capable of forming a solid core after the completion of the reaction (particularly polymerization reaction), and polymerizing the monomer to produce a light fiber. In case the light fiber is produced by such a method, it is usually important to substantially prevent air gap (also referred to as an internal defect) such as cellular defect, peel or the like from generating between the core and the clad (interface). Since such air gap lowers the light transmission efficiency of the light fiber, it becomes impossible to transfer light incident upon the core from one end to the other end with a sufficient dose of light in the comparatively long size light fiber (having a length of 20 m or more).
As the method of producing the light fiber, capable of inhibiting generation of such air gap as possible, for example, a method of filling with a monomer under pressure (disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 57-45502), a method of successively polymerizing a monomer from one end to the other end (disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 7-168029) and a method of polymerizing a monomer while applying a pressure in an axial direction of a clad (disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 7-168028). There is also known a method of covering a previously thermally-expanded clad with a jacket, polymerizing a monomer, removing the jacket, and shrinking the clad by applying heat (disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 2-306205). In the above mentioned successive polymerization method, there is usually employed a so-called batch-wise manufacturing system of filling a tubular clad, whose one end is sealed, with a monomer under pressure, fixing the clad filled with the monomer in a heating bath, and polymerizing the monomer by heating. At this time, the monomer is polymerized successively from one end to the other end by raising a heating temperature (i.e. temperature of the heating medium in the heating bath).
The formers of a series of methods described above, i.e. three methods utilizing filling with monomer under pressure, successive polymerization and axial pressurization of clad, have an effect of preventing generation of air gap at the interface between the core whose volume was reduced by polymerization, and the clad, by filling with the monomer in the amount corresponding to the volume reduction, occurred on polymerization, from the unpolymerized side.
On the other hand, the latter method has an effect of thermally expanding a clad, reversibly, and shrinking an internal radius of the clad corresponding to an external diameter of the core whose volume was reduced after polymerization, thereby enhancing the adhesion between the core and the clad.
According to the former three methods among conventional methods described above, in case the region of interface between the unpolymerized portion where the monomer retains mobility and the polymerized portion where the monomer was solidified to lose the mobility is comparatively narrow (volume reduction of the monomer occurs only within a range where an influence of the pressure from the unpolymerized portion is exerted), it is possible to fill with the monomer in the amount corresponding to the volume reduction, occurred on polymerization, from the unpolymerized side. However, these methods are not effective for the case where the light fiber is continuously produced, for example, by the steps of:
(I) transferring a clad filled with a monomer to a heating region such as temperature-controlled bath using a feeding means,
(II) initiating, carrying out and completing the reaction of the monomer in the heating region, and
(III) passing the resulting light fiber after the completion of the polymerization reaction through the heating region and winding the light fiber, using a winding means disposed outside the heating region. That is, in case the productivity is enhanced by increasing the polymerization reaction rate, it becomes impossible to obtain a sufficient monomer filling effect because the region of the interface between the unpolymerized portion and the polymerized portion (portion where the polymerization has been completed) forms a boundary portion having a certain width and the volume reduction of the monomer occurs at the portion where an influence of the pressure from the unpolymerized portion is not exerted. Accordingly, it is difficult to stably produce a light fiber which is substantially free from air gap (the number of air gap between the clad and the core, which is measured per length of 10 m, is 3 or less) in case of continuous production.
In case a batch-wise production of the light fiber is carried out by using such a method, shrinkage of the external diameter of the core is compensated by shrinkage of the core in the lengthwise direction, thereby making it difficult to produce a comparatively long size light fiber (having a length of 20 m or more).
On the other hand, in the method of covering a previously thermally-expanded clad with a jacket, completing the polymerization of a monomer, and removing air gap between the core and the clad, which accompanies the polymerization, by shrinkage of the clad with heating, the production process is complicated when continuous production is conducted. Also, in case of removing air gap, which has already been formed, in the continuous processing of a comparatively long size light fiber, there is a considerable fear that the gas from the air gap portion can not escape from the other end portion of the light fiber and is trapped in the halfway of the light fiber, resulting in formation of defects.
Thus, an object of the present invention is to provide a long size light fiber (having a length of 20 m or more), which is substantially free from air gap between the core and the clad and is superior in light transmission performance.
Another object of the present invention is to provide a method of producing a light fiber, capable of producing such a light fiber in a stable and easy manner.
According to the present invention, the problems described above can be solved by a light fiber comprising (a) a tubular clad having a predetermined length and (b) a solid core formed by reacting a filler material with which the clad is filled, the clad and the core being closely contacted with each other by shrinkage of the clad, characterized in that:
the clad is expandable under pressure,
shrinkage of the clad is initiated before the completion of the reaction of the filler material and is carried out in conformity with volume reduction of the core, which accompanies the reaction of the filler material, and
the number of air gap between the clad and the core, which is measured per length of 10 m, is 3 or less.
According to the present invention, there is also provided a method of producing a light fiber comprising (a) a tubular clad having a predetermined length and (b) a solid core formed by reacting a filler material filled in the clad, the clad and the core being closely contacted with each other by shrinkage of the clad, said method comprising the steps of:
forming the clad which is expandable under pressure,
filling the clad with the filler material and reacting the filler material in the clad while applying a pressure,
initiating a shrinking operation of the clad before the completion of the reaction of the filler material, and
shrinking the clad with volume reduction of the core, which accompanies the reaction of the filler material, thereby inhibiting generation of air gap between the clad and the core.